Voltage-controlled oscillator (VCO) with LC circuit and series resistors

ABSTRACT

A system includes a data path and a phase-locked loop (PLL) coupled to the data path. The system also includes a voltage-controlled oscillator (VCO) coupled to the PLL. The VCO includes an LC circuit with first and second differential output terminals. The VCO also includes a first resistor coupled between the first differential output terminal and drain terminals of a first pair of complementary metal-oxide semiconductor (CMOS) transistors. The VCO also includes a second resistor coupled between the second differential output terminal and drain terminals of a second pair of CMOS transistors.

BACKGROUND

The proliferation of electronic devices and integrated circuit (IC)technology has resulted in the commercialization of IC products. As newelectronic devices are developed and IC technology advances, new ICproducts are commercialized. One example IC product that is needed inelectronic devices is a voltage-controlled oscillator (VCO). In oneexample VCO, an LC circuit is combined with IC transistors to provide anoutput frequency as a function of a voltage differential.

One of the issues with LC-based VCO topologies is that the LC circuitgets clamped to the supply voltages (VDD-VSS), which degrades VCOperformance by increasing the phase noise (adding harmonic components)which up-converts 1/f noise. Also, some LC-based VCO topologies haveincreased frequency drift with respect to temperature across processcorners.

SUMMARY

In accordance with at least one example of the description, a systemcomprises a data path and a phase-locked loop (PLL) coupled to the datapath. The system also includes a voltage-controlled oscillator (VCO)coupled to the PLL. The VCO includes an LC circuit with first and seconddifferential output terminals. The VCO also includes a first resistorcoupled between the first differential output terminal and drainterminals of a first pair of complementary metal-oxide semiconductor(CMOS) transistors. The VCO also includes a second resistor coupledbetween the second differential output terminal and drain terminals of asecond pair of CMOS transistors.

In accordance with at least one example of the description, a VCOcomprises a first pair of CMOS transistors with a first terminal betweendrain terminals of the first pair of CMOS transistors. The VCO alsocomprises a second pair of CMOS transistors with a second terminalbetween drain terminals of the second pair of CMOS transistors. The VCOalso comprises a first resistor, an LC circuit, and a second resistorcoupled in series between the first and second terminals.

In accordance with at least one example of the description, anintegrated circuit comprises VCO. The VCO comprises a first differentialoutput terminal and a second differential output terminal. The VCO alsocomprises an inductor and a capacitor in parallel between the first andsecond differential output terminals. The VCO also comprises a firstresistor coupled between the first differential output terminal anddrain terminals of a first pair of CMOS transistors. The VCO alsocomprises a second resistor coupled between the second differentialoutput terminal and drain terminals of a second pair of CMOStransistors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an electrical system with avoltage-controlled oscillator (VCO) in accordance with some examples.

FIG. 2 is a block diagram showing a first LC-based VCO that does notcomply with target performance criteria.

FIG. 3 is a block diagram showing a second LC-based VCO that does notcomply with target performance criteria.

FIG. 4 is a block diagram showing a third LC-based VCO that does notcomply with target performance criteria.

FIG. 5 is a block diagram showing a fourth LC-based VCO that does notcomply with target performance criteria.

FIG. 6 is a block diagram showing an LC-based VCO that complies withtarget performance criteria.

DETAILED DESCRIPTION

Described herein is an inductor-capacitor (LC)-based VCO topologysuitable for integrated circuit (IC) implementation while achievingtarget performance criteria such as output frequency, small signal gain,and phase noise without a significant increase in complexity and cost.In some examples, an LC-based VCO with the described topology is part ofa phase-locked loop (PLL) used in a communication link device or IC witha data path. As used herein, “a communication link device” is a deviceused for data transmissions between at least two other devices. Indifferent examples, a communication link device supports wireless orwired data transmissions between the at least two other devices. In someexamples, the PLL is part of a clock and data recovery (CDR) circuitcoupled to the data path, where the VCO provides its output (e.g., adifferential signal at 6-12 GHz) to the data path.

In some examples, the VCO includes an LC circuit (L and C in parallel)with first and second differential output terminals. The VCO alsoincludes a first resistor coupled between the first differential outputterminal and drain terminals of a first pair of CMOS transistors. TheVCO also includes a second resistor coupled between the seconddifferential output terminal and drain terminals of a second pair ofCMOS transistors. The VCO also includes: a first terminal between drainterminals of the first pair of CMOS transistors; and a second terminalbetween drain terminals of the second pair of CMOS transistors. In thedescribed VCO topology, the first resistor, the LC circuit, and thesecond resistor are in series between the first and second terminals.

With the described VCO topology, IC design targets for noise, power, andarea are achieved. Example targets for the VCO include noise below −106dBc/rt (Hz) at 1 MHz offset and 12 GHz carrier, and power consumptionbelow 30 mA, which facilitates system-on-a-chip (SoC) solutions. Toprovide a better understanding, various VCO options and related issuesare described using the figures as follows.

FIG. 1 is a block diagram showing an electrical system 100 with avoltage-controlled oscillator (VCO) 118 in accordance with someexamples. In some examples, the electrical system 100 corresponds tocommunication link product that includes a system-on-a-chip (SoC) 102,where the SoC 102 includes a data path 103 with a decision circuit 106coupled to a clock and data recovery (CDR) circuit 110. As shown, theCDR circuit 110 includes a PLL 111 with a VCO 118 as described herein.In different examples, the data path 103 includes repeater components,serializer components, and/or de-serializer components. Some examplecomponents of the data path 103 includes buffers, multiplexers,demultiplexers, and control logic. Example operations of the data path103 include amplifying a received data signal 104, repeating a receiveddata signal 104, serializing a received data signal 104, orde-serializing a received data signal 104, resulting in an output signal108. Also, the output data signal 104 results from the operations of thedecision circuit 106, which is based on a clock signal (VCO_(OUT))generated by the VCO 118 described herein. Example devices or circuitswith the data path 103 and the CDR include a communication link device(e.g., a transmitter, a receiver, a transceiver, aserializer/deserializer (SERDES), or a repeater). As used herein, a“serializer/deserializer” refers to a circuit that converts data fromserial data to parallel data or vice versa. In different examples, aserializer/deserializer performs one-way conversions (e.g.,serial-to-parallel in one direction or parallel-to-serial in onedirection). In other examples, a serializer/deserializer performstwo-way conversions (e.g., serial-to-parallel in one direction andparallel-to-serial in the other direction). Other two-way conversionsare possible as well (e.g., serial-to-parallel in both directions orparallel-to-serial in both directions).

In some examples, the system 100 or SoC 102 performs transmitteroperations. In such examples, the system 100 or SoC 102 includesprocessors, sensors, and/or memory configured to provide the data signal104 to the data path 103. Additionally or alternatively, the system 100or SoC 102 performs receiver operations. In such examples, the system100 or SoC 102 includes processor and/or memory configured to receiveand process data signals from the data path 103. In the example of FIG.1, The VCO 118 is configured to provide VCO_(OUT) with a frequency thatvaries as a function of an input voltage signal (V_(CTRL)), which iscontrolled by a loop having a phase-detector 111, a loop filter 114, andthe VCO 118. As shown, VCO_(OUT) is one of the inputs to phase-detector(PD) 112 of the PLL 111. The other input to the PD 112 is provided bythe data path 103. In some examples, the loop of the PLL 111 isconfigured to adjust V_(CTRL) to maintain the frequency of VCO_(OUT) ata fixed value based on parameters such as a temperature value, a powersupply value, control signals and/or other adjustment parameters. Inother examples, the loop of the PLL 111 adjusts V_(CTRL) to vary thefrequency of VCO_(OUT) according to a particular control scheme. As anexample, the frequency of VCO_(OUT) could be varied depending on atarget data rate for the system 100, the SoC 102, or data signaling

In the example of FIG. 1, the VCO 118 includes a first pair of CMOStransistors 120 with a first terminal 122, where the first terminal 122is between the drain terminals of the first pair of CMOS transistors(see e.g., the first terminal 122A in FIG. 6). The VCO 104 also includesa second pair of CMOS transistors 124 with a second terminal 126 betweendrain terminals of the second pair of CMOS transistors (see e.g., thesecond terminal 126A in FIG. 6). The VCO 108 also includes a firstresistor (RT1), an LC circuit 128, and a second resistor (RT2) coupledin series between the first and second terminals 122 and 126. The LCcircuit 128 includes an inductor (L) in parallel with a capacitor (C),where first and second differential output terminals 130 and 132 of theVCO 108 are on opposite sides of the LC circuit 128. More specifically,RT1 is between the first terminal 122 and the first differential outputterminal 130, and RT2 is between the second terminal 126 and the seconddifferential output terminal 132. With RT1 and RT2 in series with the LCcircuit 128, surge protection is provided, which ensures the VCO 108complies with target performance criteria such as phase noise, smallsignal gain, and frequency drift as a function of temperature variation.For at least some target performance criteria, the described topologyfor the VCO 108 is an improvement over other LC-based VCO topologies.

FIG. 2 is a block diagram showing a first LC-based VCO 200 that does notcomply with target performance criteria for phase noise. In FIG. 2, thefirst LC-based VCO 200 includes a first pair of CMOS transistors (MP1and MN1) and a second pair of CMOS transistors (MP2 and MN2) in across-coupled arrangement. More specifically, with the cross-coupledarrangement, the drains of MP1 and MN1 are coupled to each other. Also,the control (gate) terminals of MP1 and MN1 are coupled to each other.Similarly, the drains of MP2 and MN2 are coupled to each other. Also,the control (gate) terminals of MP2 and MN2 are coupled to each other.As shown, the emitters of MP1 and MP2 are coupled to a supply voltage(VDD) terminal 206, where VDD=1.2V in the example of FIG. 1. Also, theemitters of MN1 and MN2 are coupled to a ground terminal 208.

In the example of FIG. 2, an LC circuit 210 is coupled between a firstdifferential output (VOP) terminal 202 and a second differential output(VON) terminal 204 of the first LC-based VCO 200. As shown, the VOPterminal 202 is coupled to the drains of MP1 and MN1, and is to thecoupled to the control (gate) terminals of MP2 and MN2. Also, the VONterminal 204 is coupled to the drains of MP2 and MN2, and to the control(gate) terminals of MP1 and MN1. In scenario 218 of FIG. 2, the firstLC-based VCO 200 is not in operation. In scenario 220 of FIG. 2, thefirst LC-based VCO 200 is clamped between a supply voltage (VDD, e.g.,1.2V provided by the VDD terminal 306) and the ground terminal 208 (VSS,e.g., 0V).

With the first LC-based VCO 200, a set of target performance parametersare considered, such as small signal noise (Gm*RP, where Gm is thetransconductance of the first LC-based VCO 200 and RP is the losses ofthe LC circuit 210), phase noise (measured using a signal-to-noise ratioor “SNR”), output swing, power consumption (V*I), and complexity. In theexample of FIG. 1, a high-voltage swing is able to reduce phase noise.When the VOP terminal 202 is at 0V and the VON terminal 204 is at 1.2Vas in scenario 220, the LC circuit 210 is clamped to the supply railsVDD and VSS via MP2 and MN1, which limits the complete operation of theLC circuit 210 and negatively impacts the phase noise and frequencystability. In sub-micron technologies with an on-chip LC circuit, thequality factor (Q) for the VCO is approximately 10, this makes the sizesof the cross-coupled CMOS pairs (MN1, MN2; and MP1 and MP2) relativelylarge. Table 1 shows simulation results for the first LC-based VCO 200.

TABLE 1 ~12 GHz VCO With L = 143 pH and Q = 10 Weak Corner Strong Corner(Simulation Results) −40° C. 125° C. −40° C. 125° C. Freq [MHz] 1145711404 11461 11587 Phase Noise @ 1 MHz offset −106.4 −106.0 −105.0 −107.6[dBc/Hz] Amplitude of Oscillation [V_(PP)] 1.03 0.97 1.39 1.34 FreqDrift with regard to temp −53 126 [MHz]In Table 1, a VCO with a 12 GHz output is assumed, where L=143 pH andQ=10. As represented in Table 1, the frequency drift in the “strongprocess corner” and the “weak process corner” are in oppositedirections. This is due to strong clamping in the strong corner. Due toat least the frequency drift across the temperature range being largerthan desired, and thus the first LC-based VCO 200 does not comply withtarget performance criteria.

FIG. 3 is a block diagram showing a second LC-based VCO 300 that doesnot comply with target performance criteria. The second LC-based VCO 300has the same cross-couple arrangement of the first pair of CMOStransistors (MP1, MN1) and the second pair of CMOS transistors (MP2,MN2) as described for the first LC-based VCO 200. As shown in FIG. 3, anLC circuit 310 is coupled between a VOP terminal 302 and a VON terminal304 of the second LC-based VCO 300. In the example of FIG. 3, resistors(e.g., RS1 and RS3) are used between a supply voltage terminal 306 andthe emitters of MP1 and MP2. More specifically, RS1 is between thesource of MP1 and the supply voltage 306, and RS3 is between the sourceof MP2 and the supply voltage terminal 306. Also, resistors (e.g., RS2and RS4) are used between the sources of MN1 and MN2 and a groundterminal 308. More specifically, RS2 is between the source of MN1 andthe ground terminal 308, and RS4 is between the source of MN2 and theground terminal 308.

With the second LC-based VCO 300, the small signal gain and the outputswing are undesirably reduced relative to the small signal gain andoutput swing of the first LC-based VCO 200 in FIG. 2. Also, the powerconsumption for the second LC-based VCO 300 is undesirably increasedrelative to the power consumption for the first LC-based VCO 200. Oneadvantage of the second LC-based VCO 300 relative to the first LC-basedVCO 200 is that phase noise is reduced. Also, the complexity of thesecond LC-based VCO 300 relative to the first LC-based VCO 200 isacceptable. Due to at least the small signal gain, output swing, andpower consumption issues, the second LC-based VCO 300 does not complywith target performance criteria.

FIG. 4 is a block diagram showing a third LC-based VCO 400 that does notcomply with target performance criteria. The third LC-based VCO 400 hasthe same cross-couple arrangement of the first pair of CMOS transistors(MP1, MN1) and the second pair of CMOS transistors (MP2, MN2) asdescribed for the first LC-based VCO 200. As shown in FIG. 4, an LCcircuit 410 is coupled between a VOP terminal 402 and a VON terminal 404of the third LC-based VCO 400. In the example of FIG. 4, resistors(e.g., RD1 and RD2) are used between the VOP terminal 402 and the drainsof MP1 and MN1. More specifically, RD1 is between the VOP terminal 402and the drain of MP1, and RD2 is between the VOP terminal 402 and theemitter of MN1. Also, resistors (e.g., RD3 and RD4) are used between theVON terminal 404 and the drains of MP2 and MN2. More specifically, RD3is between the VON terminal 404 and the drain of MP2, and RD4 is betweenthe VON terminal 404 and the drain of MN2.

With the third LC-based VCO 400, the small signal gain is the samerelative to the first LC-based VCO 200. Also, the output swing of thethird LC-based VCO 400 is undesirably decreased relative to the outputswing of the first LC-based VCO 200. Also, the power consumption of thethird LC-based VCO 400 is undesirably increased relative to the powerconsumption of the first LC-based VCO 200. One advantage of the thirdLC-based VCO 400 relative to the first LC-based VCO 200 is that phasenoise is reduced. Also, the complexity of the third LC-based VCO 400relative to the first LC-based VCO 200 is acceptable. Due to at leastthe power consumption being higher third LC-based VCO 400 does notcomply with target performance criteria as the power consumption ishigher than desired.

FIG. 5 is a block diagram showing a fourth LC-based VCO 500 that doesnot comply with target performance criteria. The fourth LC-based VCO 500has the same cross-couple arrangement of the first pair of CMOStransistors (MP1, MN1) and the second pair of CMOS transistors (MP2,MN2) as described for the first LC-based VCO 200. As shown in FIG. 5, anLC circuit 510 is coupled between a VOP terminal 502 and a VON terminal504 of the fourth LC-based VCO 500. In the example of FIG. 5, theresistance of MP1, MN1, MP2, and MN2 is adjustable as represented by theadjustable transistor circuit 512A (with MP1 and switch S1), theadjustable transistor circuit 512B (with MN1 and switch S2), theadjustable transistor circuit 514A (with MP2 and switch S3), and theadjustable transistor circuit 514B (with MN2 and switch S4). In someexamples, the size of the transistors are chosen in such a way that thetargeted output swing is reached using a calibration algorithm Byadjusting the resistance of MP1, MN1, MP2, MN2, the performance of thefourth LC-based VCO 500 relative to the first LC-based VCO 200 at thecost of increased complexity.

With the fourth LC-based VCO 500, the small signal gain, the outputswitch, and the power consumption are optimized relative to the firstLC-based VCO 200. Also, the phase noise of the fourth LC-based VCO 500is reduced relative to the phase noise of the first LC-based VCO 200.Unfortunately, the complexity of the fourth LC-based VCO 500 is highrelative to the first LC-based VCO 200. At least due to the complexityof the fourth LC-based VCO 500 and related costs, the fourth LC-basedVCO 500 does not comply with target performance criteria.

FIG. 6 is a block diagram showing an LC-based VCO 600 that complies withtarget performance criteria. The LC-based VCO 600 has the samecross-couple arrangement of the first pair of CMOS transistors (MP1,MN1) and the second pair of CMOS transistors (MP2, MN2) as described forthe first LC-based VCO 200. As shown in FIG. 6, an LC circuit 620 iscoupled between a VOP terminal 130A (an example of the firstdifferential output terminal 130 in FIG. 1) and a VON terminal 132A (anexample of the second differential output terminal 132 in FIG. 1) of theLC-based VCO 600. As shown, the LC-based VCO 600 includes a firstterminal 122A (an example of the first terminal 122 in FIG. 1) betweendrain terminals of the first pair of CMOS transistors (MP1, MN1). TheLC-based VCO 600 also includes a second terminal 126A (an example of thesecond terminal 126 in FIG. 1) between drain terminals of the secondpair of CMOS transistors (MP2, MN2). The LC-based VCO 600 also includesRT1, an LC circuit 128A (an example of the LC circuit 128 in FIG. 1),and RT2 coupled in series between the first and second terminals 122Aand 126A. More specifically, RT1 is between the first terminal 122A andthe VOP terminal 130A, and RT2 is between the second terminal 126A andthe VON terminal 132A. With RT1 and RT2 in series with the LC circuit128, surge protection is provided, which ensures the LC-based VCO 600complies with target performance criteria such as frequency drift as afunction of temperature variation, phase noise, and small signal gain.

More specifically, with the LC-based VCO 600, the small signal gain isthe same as the small signal of the first LC-based VCO 200. Also, thephase noise and power consumption of the LC-based VCO 600 are reducedrelative to the first LC-based VCO 200. One disadvantage of the LC-basedVCO 600 is that the output swing is reduced relative to the firstLC-based VCO 200. Finally, the complexity of the LC-based VCO 600 isacceptable relative to the first LC-based VCO 200. Table 2 showssimulation results for the LC-based VCO 200.

TABLE 2 Cross-Couple CMOS pairs with ~12 gHz VCO with L = 143 pH SurgeProtection and Q = 10 Weak Process Strong Process RT1 = RT2 = 8Ω CornerCorner (Simulation Results) −40° C. 125° C. −40° C. 120° C. Freq [MHz]11520 11435 11875 11865 Phase Noise @ 1 MHz offset −111.3 −109.4 −116.3−113.5 [dBc/Hz] Amplitude of Oscillation [V_(PP)] 0.83 0.76 1.21 1.14Supply Current [A] 10.1E−3 10.7E−3 26.2E−3 27.0E−3 Freq drift with temp[MHz] −85 −9In Table 2, a VCO with a 12 GHz output is assumed, where L=143 pH andQ=10. RT1 and RT2 is sized according to the minimal acceptable outputswing that can be tolerated (in this example RT1 and RT2 are set at 8Ω).As represented in Table 2, the frequency drift in the “strong processcorner” and the “weak process corner” are in the same direction. Basedon the simulation results represented in Table 2, the frequency drift ofthe LC-based VCO 600 across the temperature range is acceptable.

With the LC-based VCO 600 phase noise is reduced by at 2 dB relative tothe first LC-based VCO 200 when compared across corners withoutincreasing the current consumption. The value of RT1 and RT2 should bechosen so that RT1 and RT2 do not cause lot of degradation to outputswing, especially with slow corners where the supply voltage and MOSFETGm will be less.

In some examples, a system (e.g., the system 100 in FIG. 1) includes adata path (e.g., the data path 103 in FIG. 1), a PLL (e.g., the PLL 111in FIG. 1), and a VCO (e.g., the VCO 118 in FIG. 1, or the LC-based VCO600 in FIG. 6) coupled to the PLL. The VCO includes an LC circuit (e.g.,the LC circuit 128 in FIG. 1, or the LC circuit 128A in FIG. 6) withfirst and second differential output terminals (e.g., VOP terminal 130and VON terminal 132 in FIG. 1; or VOP terminal 130A and VON terminal132A in FIG. 6). The VCO also includes a first resistor (RT1 in FIGS. 1and 6) coupled between the first differential output terminal and drainterminals of a first pair of CMOS transistors (120 in FIG. 1, or MP1,MN1 in FIG. 6). The VCO also includes a second resistor (RT2) coupledbetween the second differential output terminal and drain terminals of asecond pair of CMOS transistors.

In some examples, the first pair of CMOS transistors includes a firstPMOS transistor (e.g., MP1 in FIG. 6) with a gate terminal coupled tothe second differential output terminal (e.g., VON terminal 132 in FIG.1, or VON terminal 132A in FIG. 6), wherein an emitter terminal of thefirst PMOS transistor is coupled to a supply voltage terminal (e.g., theVDD terminal 606 in FIG. 6). The first pair of CMOS transistors alsoincludes a first NMOS transistor (e.g., MN1 in FIG. 6) with a gateterminal coupled to the second differential output terminal, wherein anemitter terminal of the first NMOS transistor is coupled to a groundterminal (e.g., the ground terminal 608 in FIG. 6).

In some examples, the second pair of CMOS transistors includes a secondPMOS transistor (e.g., MP2 in FIG. 6) with a control (gate) terminalcoupled to the first differential output terminal (e.g., VOP terminal130 in FIG. 1, or the VOP terminal 130A in FIG. 6), wherein an emitterterminal of the second PMOS transistor is coupled to the supply voltageterminal (e.g., the VDD terminal 606 in FIG. 6). The second pair of CMOStransistors also includes a second NMOS transistor (MN2) with a gateterminal coupled to the first differential output terminal, wherein anemitter terminal of the second NMOS transistor is coupled to the groundterminal (e.g., the ground terminal 608 in FIG. 6).

In some examples, a VCO (e.g., the VCO 118 in FIG. 1, or the LC-basedVCO 600 in FIG. 6) includes a first pair of CMOS transistors (e.g., MP1,MN1 in FIG. 6) with a first terminal (e.g., 122A in FIG. 6) betweendrain terminals of the first pair of CMOS transistors. The VCO alsoincludes a second pair of CMOS transistors (e.g., MP2, MN2 in FIG. 6)with a second terminal 126A between drain terminals of the second pairof CMOS transistors. The VCO also includes a first resistor, an LCcircuit, and a second resistor coupled in series (e.g., RT1, LC circuit128A, and RT2 in series in FIG. 6) between the first and secondterminals. The VCO also includes a first differential output terminal(e.g., 130A in FIG. 6) between the first resistor and the LC circuit,and a second differential output terminal (e.g., 132A in FIG. 6) betweenthe second resistor and the LC circuit.

In some examples, a size of the first and second pairs of CMOStransistors is selected to achieve a high quality factor (e.g., 10 ormore). Also, in some examples, a supply voltage source is configured toprovide a supply voltage to the supply voltage source, where the supplyvoltage is between 1V to 1.2V. In some examples, the first and secondresistors are sized so that the VCO achieves a target small signal gainand a target phase noise. In some examples, the data path, the PLL, andthe VCO are components of an integrated circuit based on a CO21 (65 nm)process. In some examples, the small signal gain for the described VCOis greater than 1 (e.g., around 2).

With the described VCO topology, the feedback to cross-coupled CMOSpairs prevents the degradation of small signal gain. In some examples,two resistors (RT1 and RT2) are in series with the LC circuit or tank tolimit the current. The introduction of RT1 and RT2 reduces phase noiseby at least 2 dB without increasing current consumption. Also, in someexamples, the value of RT1 and RT2 is chosen such that output switchdegradation is avoided, especially with slow corner where the supplyvoltage and MOSFET transconductance will be less. With the described VCOtopology, the small signal gain is the same as the small signal of thefirst LC-based VCO 200. Also, the phase noise and power consumption ofthe described VCO are reduced relative to the first LC-based VCO 200.One disadvantage of the described LC-based VCO is that the output swingis reduced relative to the first LC-based VCO 200. Finally, thecomplexity of the described VCO is acceptable relative to the firstLC-based VCO 200. When compared with other VCO topologies, the describedVCO topology meets more target performance criteria without asignificant increase in complexity (e.g., reducing size and costcompared to the fourth LC-based VCO in FIG. 4).

The term “couple” is used throughout this description. The term maycover connections, communications, or signal paths that enable afunctional relationship consistent with this description. For example,if device A generates a signal to control device B to perform an action,in a first example device A is coupled to device B by direct connection,or in a second example device A is coupled to device B throughintervening component C if intervening component C does not alter thefunctional relationship between device A and device B such that device Bis controlled by device A via the control signal generated by device A.

Modifications are possible in the described embodiments, and otherembodiments are possible, within the scope of the claims.

What is claimed is:
 1. A voltage-controlled oscillator (VCO),comprising: a first pair of complementary metal-oxide semiconductor(CMOS) transistors with a first terminal between drain terminals of thefirst pair of CMOS transistors; a second pair of CMOS transistors with asecond terminal between drain terminals of the second pair of CMOStransistors; a first resistor, an LC circuit, and a second resistorcoupled in series between the first and second terminals.
 2. The VCO ofclaim 1, further comprising: a first differential output terminalbetween the first resistor and the LC circuit; and a second differentialoutput terminal between the second resistor and the LC circuit.
 3. TheVCO of claim 2, wherein the first pair of CMOS transistors comprises: afirst PMOS transistor with a gate terminal coupled to the seconddifferential output terminal, wherein an emitter terminal of the firstPMOS transistor is coupled to a supply voltage terminal; and a firstNMOS transistor with a gate terminal coupled to the second differentialoutput terminal, wherein an emitter terminal of the first NMOStransistor is coupled to a ground terminal.
 4. The VCO of claim 3,wherein the second pair of CMOS transistors comprises: a second PMOStransistor with a gate terminal coupled to the first differential outputterminal, wherein an emitter terminal of the second PMOS transistor iscoupled to the supply voltage terminal; and a second NMOS transistorwith a gate terminal coupled to the first differential output terminal,wherein an emitter terminal of the second NMOS transistor is coupled tothe ground terminal.
 5. The VCO of claim 3, wherein a supply voltageprovided by the supply voltage terminal is between 1V to 1.2V.
 6. TheVCO of claim 1, wherein a size of the first and second pairs of CMOStransistors is selected to achieve a quality factor of at least
 10. 7.The VCO of claim 1, wherein the VCO is part of a phase-locked loop (PLL)of a communication link device.
 8. The VCO of claim 1, wherein the firstand second resistors are sized so that the VCO achieves a small signalgain greater than 1.